1. Field of Invention
The present invention relates to a beam homogenizer which homogenizes the energy distribution of a beam spot on an irradiation surface by using a more compact optical system, and also relates to a laser irradiation apparatus using the beam homogenizer. More specifically, the present invention relates to a beam homogenizer which homogenizes the energy distribution of a beam spot on an irradiation surface by using a more compact optical system obtained by decreasing the distance between lenses to shorten optical path length, and also relates to a laser irradiation apparatus using the beam homogenizer.
2. Description of the Related Art
In recent years, research has been widely conducted on techniques for carrying out laser annealing to a non-single crystal semiconductor film (an amorphous semiconductor film or a semiconductor film having crystallinity which is not single crystal, such as poly-crystal or microcrystal) formed over an insulating substrate such as a glass substrate. The laser annealing described herein indicates a technique for recrystallizing an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, and a technique for crystallizing a non-single crystal semiconductor film formed over a substrate. Moreover, the laser annealing includes a technique applied to modification or flattening of a surface of a semiconductor substrate or a semiconductor film, a technique in which laser irradiation is carried out after introducing a crystallization-promoting element such as nickel into an amorphous semiconductor film, a technique in which a semiconductor film having crystallinity is irradiated with a laser beam, and so on.
The laser annealing is employed for the crystallization because a glass substrate has a low melting point and a glass substrate is deformed when substrate temperature gets too high at the annealing. In contrast, a laser can give high energy only to a non-single crystal semiconductor film without changing the temperature of a substrate that much. As a laser annealing method, the following method is often employed because of its high mass productivity and industrial superiority: a laser beam emitted from a pulsed laser with high output power such as an excimer laser is shaped into a square spot with several centimeters on each side or into a rectangular spot with a length of 10 cm or more in a long-side direction on an irradiation surface through an optical system and an irradiation position of the beam spot is moved relative to the irradiation surface. It is to be noted that among the rectangular beam spots, a rectangular beam spot having a particularly high aspect ratio is referred to as a linear beam spot in this specification.
In particular, unlike a punctate beam spot requiring to be moved from front to back and from side to side, a linear beam spot can provide high mass productivity because a large irradiation surface can be irradiated with the linear beam spot only by moving the linear beam spot in one direction perpendicular to a direction along a longer width of the linear beam spot. The laser beam is moved in the direction (hereinafter referred to as a short-side direction) that is perpendicular to the direction (hereinafter referred to as a long-side direction) along the longer width of the linear beam spot since this is the most effective scanning direction. Because of such high mass productivity, current laser annealing is mainly employing a linear beam spot obtained by shaping a beam spot of a pulsed excimer laser through an appropriate optical system.
FIGS. 6A and 6B show an example of an optical system for changing a sectional shape of a beam spot into a linear shape on an irradiated surface. The optical system shown in FIGS. 6A and 6B is an extremely general optical system. This optical system not only changes the sectional shape of the beam spot into a linear shape but also homogenizes the energy distribution of the beam spot on the irradiation surface at the same time. Generally, the optical system for homogenizing the energy distribution of the beam spot is referred to as a beam homogenizer.
The optical system shown in FIGS. 6A and 6B is also a beam homogenizer. In the case of using a XeCl excimer laser (wavelength 308 nm) as a light source, it is preferable to use quartz as a base material of the optical system. If another excimer laser with a shorter wavelength is used as a light source, it is preferable to use fluorite, MgF2, or the like as the base material in order to obtain high transmittance.
FIG. 6A is a side view of a beam homogenizer for forming a linear beam spot. The side view includes in its paper a short-side direction of the linear beam spot formed by the beam homogenizer. A spot of a laser beam emitted from a laser oscillator 601, which is a XeCl excimer laser, is divided in one direction by cylindrical lens arrays 602a and 602b. If a mirror is inserted in the middle of the optical system, the short-side direction is bent in a direction of light bent by the mirror.
In this structure, the beam spot is divided into four beam spots. These spots are combined into one spot once by a cylindrical lens 604. The beam spots that have separated again are reflected on a mirror 606 and then condensed into one beam spot again by a doublet cylindrical lens 607 on an irradiation surface 608. The doublet cylindrical lens is a lens including two cylindrical lenses. This homogenizes the energy of the linear beam spot in its short-side direction and determines the length of the linear beam spot in its short-side direction.
FIG. 6B is a plan view of the beam homogenizer for forming the linear beam spot. The plan view includes in its paper a long-side direction of the linear beam spot formed by the beam homogenizer. The spot of the laser beam emitted from the laser oscillator 601 is divided in a direction perpendicular to the long-side direction by a cylindrical lens array 603. If a mirror is inserted in the middle of the optical system, the long-side direction is bent in a direction of light bent by the mirror.
In the structure of this lens array 603, the beam spot is divided into seven beam spots. After that, the seven beam spots are combined into one spot by a cylindrical lens 605 on the irradiation surface 608. An optical path after the mirror 606 is shown with dotted lines, and moreover the dotted lines show correct optical path and position of the irradiation surface in the case of not providing the mirror 606. This homogenizes the energy of the linear beam spot in its long-side direction and determines the length of the linear beam spot in its long-side direction.
As mentioned above, the cylindrical lens array 602a and the cylindrical lens array 602b and the cylindrical lens array 603 serve as lenses for dividing the spot of the laser beam. The homogeneity of the energy distribution of the obtained linear beam spot is determined based on the number of the divided beam spots.
In general, an excimer laser emits a rectangular laser beam having an aspect ratio of approximately 1 to 5. The spot of the laser beam has Gaussian intensity distribution where the intensity is higher toward the center of the spot. The optical system shown in FIGS. 6A and 6B changes the beam spot into a linear beam spot having homogeneous energy distribution with a size of 320 mm×0.4 mm.
The linear beam spot shaped by the above structure is delivered as being overlapped in such a way that the linear beam spot is displaced gradually in the short-side direction of the linear beam spot. Then, the laser annealing can be carried out to the whole surface of a non-single crystal semiconductor film so as to crystallize it or to enhance its crystallinity. In mass-production factories, currently, laser annealing is performed to semiconductor films using a linear beam spot shaped by the optical system as above. It is to be noted that some beam homogenizers include a reflecting mirror (for example, see Patent Document 1: Japanese Published Patent Application Laid-Open No. 2001-291681).
For several years, the size of a glass substrate has increased rapidly in a semiconductor device manufacturing process in order to form more semiconductor devices with one substrate and to improve mass productivity. With the increase in the size of the glass substrate, the improvement of laser annealing process capability by means of the extension of a linear beam in its long-side direction has been more strongly demanded. However, the extension of a linear beam in its long-side direction causes a problem in that the size of an optical system for forming the linear beam increases with the extension of the linear beam in its long-side direction, thereby increasing the area occupied by the optical system.